Odd. If there is a leak you would probably smell it all the time. Maybe RSVP canister or purge solenoid problem?
My Jeep: 2001 Grand Cherokee Limited, 245/65R17 Falken Rocky Mountain ATS, K&N drop in air filter, Bilstein HD's. De-badged, eagle-eye LED Tails & LED 3rd brake light, Addco front and rear sway bars, Superchips tuned & TM snip, interior 6k LED's, Gen3 ccfl projectors with Volt 6k HID lows, akebono brake upgrade with R1 Concepts premium rotors.